1. Field of the Invention
The present invention relates to an apparatus for photographing a moving body such as a vehicle. More particularly, the present invention relates to photographing a character sign provided on a moving body, such as a license plate of a vehicle, a shape of a moving body and a driver of a moving body.
2. Description of the Prior Art
An apparatus for photographing a license plate of a vehicle has been developed for the purpose of measuring a time taken to drive between or among a plurality of spots or monitoring vehicles breaking a speed limit, etc. An apparatus disclosed in Japanese Patent Laid-Open Publication No. Hei 5-325091 supports at least two cameras above a road using upper arms of installed poles. These cameras are situated on the downstream of an approaching vehicle in the direction of traffic flow. In addition, one of these cameras detects the approaching vehicle while the other takes a picture of a license plate of the approaching vehicle. Capture zones of the detection camera and the enforcement camera are so determined that the capture zone of the detection camera is positioned further upstream than that of the enforcement camera. When the entering of the vehicle driving on a road into the capture zone of the detection camera is detected on the basis of a result of the photographing by the detection camera, the enforcement camera carries out the photographing operation. A picture of a license plate of the approached vehicle can be consequently taken. It should be noted that the detection camera in this apparatus is just an example of a vehicle detector.
Since a response delay is generated in every kind of vehicle detector, a vehicle moves to some extent during this response delay. Here, the enforcement camera has a certain angle of visibility (the capture zone). Therefore, if the moving distance of a vehicle within a time of response delay by the vehicle detector is short enough, namely, as long as the position of the vehicle after the movement response delay is still within the capture zone of the enforcement camera, a picture of the license plate of the vehicle can be taken.
The moving distance of the vehicle during the response delay of the vehicle detector is determined by a speed of the vehicle. If the vehicle moves outside the capture zone of the enforcement camera during a time period (=a time of the response delay of the vehicle detector) from the actual approach of the vehicle until the approach of the vehicle is detected by the vehicle detector, a picture of the license plate can not be taken, as shown in FIG. 18. In FIG. 18, the capture zone of the enforcement camera extends from a position L1 to a position V along the direction of the vehicle's movement, and a vehicle speed at which a picture of the license plate can be barely taken, i.e., a vehicle speed at which the vehicle having passed the position L1 takes a time equal to a response delay time of the vehicle detector to arrive at the position V, is represented as S. That is, a picture of the license plate of the vehicle whose speed exceeds S can not be taken by the apparatus disclosed in Japanese Patent Laid-Open Publication No. Hei 5-325091.